That seems off. A quick search shows New Ghis as bringing four legions to the fight – the wiki says there’s six, so I may have missed two – but I don’t see anywhere in the text saying a legion is 6,000 men. Given that the Unsullied are sold in thousands or in centuries, and the story of the 3,000 of Qohor, I’d lean more to a legion being 3,000 men at most. That would suggest that New Ghis has 18,000 men maximum.
As for the Yunkish numbers, the wiki seems to be taking an average from a very blanket statement that of the 40 commanders,“some had twenty soldiers, some two hundred or two thousand.” To me, that statement is meant to suggest extreme variability and a hierarchy between the greater and middling and lesser slave owners (which would make sense, given the extreme inequalities of wealth at the top found within slave societies), so I would be very hesitant to say the average is ~750.